1. Field of the Invention
The present invention relates to pellicles for protecting photomasks from particulate contamination and more particularly to pellicles suitable for use with mid and deep ultraviolet radiation.
2. Description of the Prior Art
Pellicles are thin, transparent membranes or films generally manufactured from a polymeric material. Their function is to act as a dust cover and to hold particulate matter outside of the focal plane of an optical apparatus.
Pellicles are widely used in the integrated circuit manufacturing process both to protect photomasks from particulate contamination and to extend the mask life. The importance of pellicles to semiconductor manufacturers is reviewed in an article by Ron Iscoff. See R. Iscoff, Pellicles 1985: An Update, Semiconductor International (Apr. 1985).
The use of pellicles in conjunction with a projection printing system was described by Shea, et al in U.S. Pat. 4,131,363, issued Dec. 26, 1978. More recently, a broad class of pellicles and a method for forming these pellicles was described by Winn in U.S. Pat. Nos. 4,378,953 and 4,536,240, issued on Apr. 5, 1983 and Aug. 20, 1985, respectively.
Currently, the material most widely used in pellicle manufacturing is nitrocellulose. As a raw material, the attractiveness of nitrocellulose is reduced because it is highly flammable and must be stored in a wetted condition. Additionally, nitrocellulose is somewhat hygroscopic and this property makes manufacturing under humid conditions difficult. As a finished product, nitrocellulose pellicles wrinkle when wetted with water thus making cleaning or storing under humid conditions difficult.
A more important problem with nitrocellulose pellicles is that nitrocellulose does not transmit ultraviolet (UV) light very well. Below two hundred and sixty nanometers, nitrocellulose transmits less than seventy percent (70%) of incident light. This limitation in a nitrocellulose pellicle (and also in mylar pellicles) was documented by R. Hershel in Pellicle Protection of IC Masks, Report by Hershel Consulting, Inc. (August 1981). Additionally, the irradiation of nitrocellulose pellicles with ultraviolet light causes the pellicle to become discolored thus further reducing its transparency.
The need for pellicles which transmit UV light arises because improvements in the lithographic process used in manufacturing integrated circuits are dependent on reducing the wavelength of the incident light used. One project undertaken to develop a broadband pellicle capable of transmitting UV light is described by I.E. Ward and D.L. Duly in Optical Microlithography III: Technology for the Next Decade, SPIE, Vol. 470, pp. 147-154 (H.L. Stover - Editor) (1984). This paper also discloses the use of an unspecified antireflective layer coated on one or both sides of the pellicle to reduce optical interference.
Optical interference is caused by internal reflection of light within the pellicle and is evidenced by oscillating behavior in the transmission spectrum of the pellicle. Current approaches to dealing with this problem include applying antireflective (AR) coatings to the pellicle and controlling the thickness of the pellicle. As noted by Hershel and Ward and Duly, supra, a problem with AR coatings is that they do not adhere well to the pellicle surface. This imperfect adhesion results in cracking and flaking of the AR coating which ruins the pellicle.
In U.S. Pat. No. 4,657,805, issued Apr. 14, 1987, Fukumitsu, et al disclose the use of thin fluoropolymer films as antireflective layers for a pellicle. Multiple layers of the fluoropolymer films are coated on a core layer pellicle to form a five-layer pellicle structure with the indexes of refraction of the various layers being chosen to reduce internal reflection and scattering.
The UV transmitting pellicles of Ward are described more completely in a series of three patents. U.S. Pat. No. 4,482,591, issued Nov. 13, 1984 discloses a pellicle comprised of polyvinyl butyral resin (PBR) and the use of a ring with an adhesive side to remove the pellicle from a wafer. U.S. Pat. No. 4,499,231, issued Feb. 12, 1985 discloses a pellicle comprising PBR and a dispersion of colloidal silica. U.S. Pat. No. 4,476,172, issued Oct. 9, 1984 discloses pellicles comprised of a PBR derivative that includes a silane moiety.
Problems also exist in the processes used to manufacture pellicles. For example, typically, a pellicle is formed by depositing a polymer solution on an inert substrate and then evaporating the solvent. This leaves the pellicle coated on the inert substrate. Removing the delicate pellicle from the substrate is a difficult, but necessary step in the process. U.S. Pat. No. 4,536,240 by Winn, discloses a method for accomplishing this task by bonding a frame to the pellicle and then peeling the pellicle off the substrate. In conjunction with this procedure, a suitable release agent can be applied to the substrate prior to applying the polymer solution and aid in removing the pellicle. Using this procedure, however, still results in many pellicles being ripped during the removal step.
Duly, et al., in U.S. Pat. No. 4,523,974, issued June 18, 1985, disclose a method for manufacturing a pellicle from polymethylmethacrylate (PMMA) that includes the steps of applying a gold film to the surface of an oxidized wafer, coating a thin layer of PMMA on the gold film, removing the PMMA and gold layers from the wafer and etching off the gold layer.